For the production of highly elastic polyurethane cold cured foams, an at least difunctional polyisocyanate, such as toluene diisocyanate or diphenylmethane diisocyanate is reacted with a polyol which has at least two hydroxyl groups per molecule and, on the average, a high proportion of primary hydroxyl groups. Such polyols are usually prepared by first adding propylene oxide to a starter alcohol and then adding ethylene oxide to this addition product in such amounts that at least 40% of the hydroxyl groups, and preferably 70 to 90% of the hydroxyl groups, are present in the form of primary hydroxyl groups.
Due to the high content of primary OH groups, these polyols exhibit a high reactivity towards isocyanates. In contrast to conventional polyurethane foams, the so-called hot cured foams, a high cross-linking density is therefore achieved already during the foaming process. This has the advantage that curing takes place without supplying external energy and that the time required for the curing as a whole is reduced. It is, however, a disadvantage that the tendency to form closed-cell foams is increased and that there is less processing leeway. Processing leeway is understood to be the tolerance limits within which it is possible to deviate from a formulation without endangering the formation of stable and, at the same time, open-celled foams.
The reduced leeway which results from the high reactivity of the foaming components and with that the narrow range for forming a stable, but still open-celled foam, does not permit those products to be used as foam stabilizers, which have been used successfully for the production of so-called hot cured foams. Moreover, such stabilizers would lead to foams with an undesirably fine and regular pore structure.
To obtain cold hardening, highly elastic polyurethane foam bodies with satisfactory application-technical characteristics, two principal methods have been developed from a practical point of view, of which there are numerous variations.
Pursuant to the first method, a procedure is used wherein higher functional isocyanates are employed which, on the average, have about 2.2 to 2.6 isocyanate groups. These isocyanates are usually reacted with trifunctional polyols which have a high content of primary hydroxyl groups as well as with low molecular polyfunctional cross-linkers, such as glycerin or triethanolamine. Due to the high reactivity of the raw materials for the foaming, and caused by the use of the low molecular polyfunctional cross-linkers, a chemically stabilized foam body is obtained in principle even without use of foam stabilizers. The foam bodies obtained, however, exhibit an irregular cell structure, wherein the individual cells are essentially closed. This, in turn, means that the foam bodies or materials are not technically usable.
In this first method, it is therefore necessary to add substances to the reaction formulation which render the cell structure uniform, but which do not additionally stabilize the foam from a physical point of view and, thus, cause closing of the cells. This result, pursuant to U.S. Pat. No. 4,042,540, is accomplished by also using organopolysiloxanes of the general formula. ##STR2## In this formula, R.sup.1 stands for hydrocarbon groups with, preferably, 90% of the R.sup.1 groups being methyl. Other examples of alkyl groups are, among others, halogen alkyl groups, such as chloromethyl- or 3-chloropropyl groups. The patent also mentions that R.sup.2 may be the same as R.sup.1. Of particular importance is, however, that the subscript n has a value of from 2 to 10 and that the organopolysiloxane is not permitted to contain units whose value exceeds n=10. This, of course, means that the patent is not concerned with equilibrated mixtures with a mole weight distribution which follows statistical laws, such as is common in organosilicon chemistry, but rather the patent is concerned with mixtures from which the units of higher molecular weight are separated by special measures. This, for example, may be accomplished by fractional distillation so that the polysiloxane mixture, subsequent to the separation, exclusively contains siloxanes with 2 to at the most 12 silicone atoms. The preparation of such a polysiloxane cut is cumbersome, requiring considerable expenditure and work, causes loss of substance and, moreover, causes considerable expense, thus increasing the price of the foam bodies.
This first-mentioned basic method has a further disadvantage. This additional disadvantage resides in the fact that the method can only be carried out within very narrow process parameters. In other words, there is very little leeway to vary the method conditions. This in turn means that the desired highly elastic foams with controlled cell structure are only obtained if very narrow formulation limits are maintained. Even then, the physical characteristics of these foam bodies are not satisfactory for a multitude of applications. Due to their high extent of cross-linking, the foam bodies have very low values for elongation at break and tear resistance which render them unsuitable for many applications.
Turning now to the second basic method, this procedure was developed in order to overcome the above-mentioned disadvantages. This method alternative resides in the use of reaction partners which have a lesser cross-linking effect. However, the foam which is formed in this procedure has to be physically stabilized in order to prevent a collapse or relapse of the foam and in order to obtain uniform cell structures.
This second method of operation is carried out, in addition to the reactive polyols, with predominantly difunctional isocyanates, such as pure toluene diisocyanate or mixtures of toluene diisocyanate with crude diphenylmethane diisocyanate in a weight ratio of toluene diisocyanate to diphenylmethane diisocyanate of at least 80% by weight to at the most 20% by weight. In some process variations, the moiety of lower molecular cross-linkers is decreased. In order to improve the hardness of the foam bodies to be obtained, not only polyols composed of propylene oxide and ethylene oxide are used, but also such polyols which additionally contain polymeric components which are chemically bound or physically dispersed into the system. Examples for this are, for example, polymers of acrylonitrile and styrene as well as polymeric urea derivatives. Particularly, the polymeric urea derivatives act on the cell walls in the manner of solid particles and cause the desired cell opening.
As stated, this second method of operation requires stabilization of the foam and, for this purpose, special foam stabilizers are required in order to compensate for the lack of inherent stability of the foams. Two groups of stabilizers are known for this purpose from the state of the art.
One of those groups comprises polysiloxane-polyoxyalkylene-mixed block polymers whose polysiloxane block (or blocks) have an average molecular weight of 150 to 1500 and whose polyoxyalkylene block (or blocks) have an average molecular weight of 150 to 1500. The mixed block polymers are free from hydroxyl groups. Such products and their use are disclosed in, for example, U. S. Pat. No. 3,741,917 and No. 4,031,044.
The second group of stabilizers is formed of polysiloxanes which have at some of the silicon atoms Si-C bound organic groups. Such groups are:
______________________________________ Cyanoalkyl disclosed in U.S. Pat. No. 3,952,038, Cyanoalkoxyalkyl disclosed in Can. Pat. No. 1,032.551, Sulfolanyloxyalkyl disclosed in U.S. Pat. No. 4,110,272, Morpholinoalkoxyalkyl disclosed in U.S. Pat. No. 4,067,828, Tert. Hydroxylalkyl disclosed in U.S. Pat. No. 4,039,490, Chloromethyl disclosed in Eur. Pat. No. 0 000 761. ______________________________________
Siloxanes with such organofunctional groups, however, are not readily accessible and, moreover, are not fully satisfactory in respect to their stabilizing effects. The starting materials required for their synthesis can be obtained, on a technical scale, with great difficulties only and, in some instances, are not available at all.
For example, if one desires to produce chloromethyl group containing polysiloxanes, it is necessary to have access to monofunctional dimethyl chloromethyl chlorosilane (I) and, particularly, difunctional methyl-chloromethyl dichlorosilane (II): ##STR3##
The monofunctional silane I can be obtained by photocatalytic halogenation of trimethylchlorosilane. However, the analogous synthesis of the difunctional silane II causes considerable difficulties. Therefore, .alpha., .omega.-chloromethyl-substituted methylpolysiloxanes are only commercially available. At this point, the present invention sets in. The present invention has, as its primary task, to provide superiorly active stabilizers to use in connection with the above-described second method of operation for the production of cold hardening, highly elastic polyurethane foams, such superiorly acting stabilizers to be readily accessible and resulting in the production of foams or foam bodies with highly satisfying application-technical characteristics.
Surprisingly it has been found that the above requirements are fully met by polysiloxanes having chloropropyl groups linked to the Si atom. Chloropropyl-substituted polysiloxanes are known in the art and are readily available and can be produced in an economic manner. It is possible to produce also such siloxanes in an economic manner in which also the difunctional (chain-forming) siloxy units exhibit chloropropyl groups. In this manner, these modified siloxanes are more readily and in a simpler manner adapted to the conditions which are prescribed or which are the corollary of the formulation and characteristics. In addition, they have improved stabilizing characteristics.